The present invention relates to rotary cranes in general, and more particularly, to the rotary crane having a telescopic mast carrying a foldable boom at the upper end thereof.
Rotary cranes of the type under discussion include a boom formed of a number of hinged elements each formed of a frame of a triangular shape, which elements are hingedly connected to each other to form the boom which may be folded up or expanded. Each hinged element may be formed of two lower chords and one upper chord. The chords are connected to each other by rods. The crane is normally provided with a rope system.
Rotary cranes of the type under consideration and having three hinged elements have a relatively large working zone, for example about 45 meters.
The disadvantage of these otherwise satisfactory rotary cranes is that they have large width when transported on highways if the boom tip is not disassembled from the remaining hinged portions of the boom.
It should be also noted that with the known constructions the raising of the crane with a twofold support of the boom and where the end hinge element and the intermediate element each has an individual boom supporting rope is complicated. The raising of the crane has been up till now performed in such a fashion that the mast was first lifted and than its upper telescopic portion was moved in the upward direction. Then the boom tip was released from the anchorage and lifted from the hinge on the mast and released from the ground by hand of an operator. The upper portion of the mast was again moved further in the upward direction and the intermediate hinged element was pivoted, connected to the end hinged element and both elements were aligned by hand in the vertical direction. The upper portion of the mast then was lowered unless the intermediate element has been positioned near the ground. After that, also by hand with the aid of two operator forces, the boom tip was attached to the intermediate element of the boom by bolts. The inner mast portion was then fully raised upwardly and the boom was lifted to a horizontal position by means of the boom supporting rope, a motor and a rope drum driven by a tackle. The above described operation requires manual labor and is extremely time consuming. In the known mode of operation of the crane the inner mast must be raised and descended for several times. Thus the known structures have been considered as troublesome and complex.
The crane described in German Pat. No. 20 20 238 has a boom comprised of two elements. This crane has a folding rope fixed to the lower boom element and directed to a deflection roll mounted on the telescopic mast. The boom can be pivoted, folded and expanded without shortening or lengthening of the folding rope during the raising of the upper portion of the telescopic mast.